Tunable reflectors based on multi-cavity interference

ABSTRACT

A reflective structure includes an input/output port and an optical splitter coupled to the input/output port. The optical splitter has a first branch and a second branch. The reflective structure also includes a first resonant cavity optically coupled to the first branch of the optical splitter. The first resonant cavity comprises a first set of reflectors and a first waveguide region disposed between the first set of reflectors. The reflective structure further includes a second resonant cavity optically coupled to the second branch of the optical splitter. The second resonant cavity comprises a second set of reflectors and a second waveguide region disposed between the second set of reflectors.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. No.13/608,920, filed on Sep. 10, 2012, entitled “Tunable Reflectors BasedOn Multi-Cavity Interference,” which application claims priority to U.S.Provisional Patent Application No. 61/532,177, filed on Sep. 8, 2011,entitled “Tunable Multi-Interference Loop,” and U.S. Provisional PatentApplication No. 61/613,446, filed on Mar. 20, 2012, entitled “ReflectionSystem including Y-Junction Cavities,” the disclosures of which arehereby incorporated by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

Silicon photonics is an emerging technology that enables integration ofmany optical components on a single chip. One of the sets of desirableelements provided by silicon photonics are tunable reflectors, which canbe utilized as an enabling technology for a wide variety ofapplications, particularly for laser feedback.

Despite the progress made in silicon photonics, there is a need in theart for improved methods and systems related to tunable reflectors.

SUMMARY OF THE INVENTION

Embodiments of the present invention relate to optical systems. Moreparticularly, embodiments of the present invention relate to tunablereflectors utilizing resonant cavities. As an example, some embodimentsof the present invention relate to coupled Fabry-Perot cavities. Otherembodiments relate to optical reflection systems utilizing Y-junctioncavities. Various embodiments of the present invention relate to devicesthat can provide a predetermined reflection coefficient at a specificwavelength using the Vernier effect. Tunability is provided, enablingimplementation in applications including reflectors for laser feedback.

According to an embodiment of the present invention, a reflectivestructure is provided. The reflective structure includes an input/outputport and an optical splitter coupled to the input/output port. Theoptical splitter has a first branch and a second branch. The reflectivestructure also includes a first resonant cavity optically coupled to thefirst branch of the optical splitter. The first resonant cavitycomprises a first set of reflectors and a first waveguide regiondisposed between the first set of reflectors. The reflective structurefurther includes a second resonant cavity optically coupled to thesecond branch of the optical splitter. The second resonant cavitycomprises a second set of reflectors and a second waveguide regiondisposed between the second set of reflectors.

According to another embodiment of the present invention, a tunablereflector structure is provided. The tunable reflector includes aninput/output port and an optical splitter coupled to the input/outputport. The tunable reflector also includes a first resonant cavitycoupled to a first output of the optical splitter and a second resonantcavity coupled to a second output of the optical splitter. At least oneof the first resonant cavity or the second resonant cavity include aphase control element and the first resonant cavity and the secondresonant cavity form a loop with the optical splitter.

According to an alternative embodiment of the present invention, areflective structure is provided. The reflective structure includes aninput/output port and a Y-junction waveguide optically coupled to theinput/output port. The Y-junction waveguide includes a primary armincluding a primary propagation section, a first primary reflectorcoupled to the primary propagation section, a primary Y-junctionwaveguide coupled to the primary propagation section, and a secondprimary reflector coupled to the primary Y-junction waveguide. TheY-junction waveguide also includes a secondary arm including a secondarypropagation section, a first secondary reflector coupled to thesecondary propagation section, a secondary Y-junction waveguide coupledto the secondary propagation section, and a second secondary reflectorcoupled to the secondary Y-junction waveguide.

According to an embodiment of the present invention, a reflectivestructure includes an input/output port and a Y-junction waveguideoptically coupled to the input/output port. The Y-junction waveguideincludes a primary arm including a primary propagation section, a firstprimary reflector coupled to the primary propagation section, a primaryY-junction waveguide coupled to the primary propagation section, and asecond primary reflector coupled to the primary Y-junction waveguide.The Y-junction waveguide also includes a secondary arm including asecondary propagation section, a first secondary reflector coupled tothe secondary propagation section, a secondary Y-junction waveguidecoupled to the secondary propagation section, and a second secondaryreflector coupled to the secondary Y-junction waveguide.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide a compact tunable reflector, compatible with thin(e.g., ˜200 nm) Silicon on Insulator (SOI), that may be tuned (e.g.,thermally) with relatively low power. This is in contrast to binarysuperimposed grating that are typically very long and compatible withthick (e.g., 1.5 μm) silicon. Also, embodiments of the present inventionprovide an easier fabrication process than conventional techniques.These and other embodiments of the invention along with many of itsadvantages and features are described in more detail in conjunction withthe text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified plan view of a tunable reflective structureaccording to an embodiment of the present invention;

FIG. 1B is a plot showing reflectance spectra for cavities of thetunable reflective structure illustrated in FIG. 1A;

FIG. 1C is a plot showing reflectance spectra for the tunable reflectivestructure illustrated in FIG. 1A;

FIG. 1D is a plot showing spectral tuning of the reflectance spectra forcavities of the tunable reflective structure illustrated in FIG. 1A;

FIG. 1E is a plot showing spectral tuning of the reflectance spectra forthe tunable reflective structure illustrated in FIG. 1A;

FIG. 2A is a simplified plan view of a tunable reflector according to anembodiment of the present invention;

FIG. 2B is a plot showing reflectance spectra as a function of indexshift for the tunable reflector illustrated in FIG. 2A;

FIG. 2C is a plot showing reflectance spectra over a second range forthe tunable reflective structure illustrated in FIG. 2A;

FIG. 3A is a simplified plan view of a tunable reflector according to analternative embodiment of the present invention;

FIG. 3B is a plot showing reflectance spectra for the tunable reflectorillustrated in FIG. 3A;

FIG. 4A is a simplified plan view of a multi-cavity reflector accordingto an embodiment of the present invention;

FIG. 4B is a plot showing reflectance spectra for individual andcombined cavities as illustrated in FIG. 4A;

FIG. 4C is a plot showing reflectance spectra for individual andcombined cavities with phase control according to an embodiment of thepresent invention;

FIG. 5 is a simplified flowchart illustrating a method of operating atunable reflector according to an embodiment of the present invention;

FIG. 6 is a simplified flowchart illustrating a method of providingvariable spectral reflectance according to an embodiment of the presentinvention;

FIG. 7 is a simplified schematic diagram illustrating a laserincorporating reflective structures provided by embodiments of thepresent invention;

FIG. 8A is a cross-sectional diagram of a waveguide structure at a firstposition according to an embodiment of the present invention;

FIG. 8B is a cross-section diagram of a waveguide structure at a secondposition according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Silicon photonics is an emerging technology that enables integration ofmany optical components on a single chip. Tunable reflectors are of highimportance for such a technology for a wide variety of applications, andparticularly for laser feedback. Having a hybrid system with anintegrated laser on a silicon chip is helpful or essential for thistechnology to be useful. According to embodiments of the presentinvention, wavelength tunable reflective structures are provided thatare suitable for integration with a silicon-on-insulator (SOI) platform.

FIG. 1A is a simplified plan view of a tunable reflective structure 100according to an embodiment of the present invention. Referring to FIG.1A, a first example configuration for a tunable reflector is provided.The input waveguide 110 is split by a Y-junction 112, with each branchof the Y-junction being incident on a side-coupled cavity, respectively.In other words, the input ports to each waveguide, which areside-coupled to cavities with different lengths, are combined using aY-junction. As illustrated in FIG. 1A, first side-coupled cavity 120 ischaracterized by a central length of 149.3 μm and second side-coupledcavity 130 is characterized by a central length of 138.1 μm. Firstside-coupled cavity 120 includes mirrors 124 and 126, which can bedistributed feedback (DFB) gratings, Bragg gratings, sidewall modulatedwaveguides, or other suitable reflectors. If periodic reflectors areused for these mirrors, strong reflection occurs at wavelengths withinthe stopband of the reflectors. Wavelengths outside this stopband willbe transmitted through the reflectors, where the light may be scatteredout using a suitable waveguide termination so as to avoid reflection.

Second side-coupled cavity 130 includes mirrors 134 and 136, which canbe DFB gratings or other suitable reflectors. It should be noted thatthe embodiment of the present invention illustrated in FIG. 1A utilizesresonant cavities 120 and 130 coupled to the branches of the opticalsplitter 112 by directional couplers. Thus, this design differs from anoptical splitter joined to DBR mirrors (or binary superimposedgratings), which, although they provide feedback based on Braggreflections, are not resonant structures. The resonant structuresdescribed herein provide a reflectance spectra with a comb offrequencies associated with the free spectral range of the resonantcavity that differs from reflection from a grating structure.

When both cavities are resonant, strong reflection at the input 110occurs, producing reflection 111. In the configuration illustrated inFIG. 1A, the coupling region from the input waveguide 110 to each ofcavities 120 and 130 is aligned with the cavity center, although this isnot required by the present invention.

A phase shifter 140 is provided in the waveguide disposed between theinput waveguide 110 and the first side-coupled cavity 120. In someimplementations, an additional phase shifter (not shown) is provided inthe waveguide disposed between the input waveguide 110 and the secondside-coupled cavity 130. Additionally, control over the index ofrefraction in the side-coupled cavities 120 and 130 is provided by theintegration of phase control sections 122 and 132, respectively.

Referring once again to FIG. 1A, a set of Fabry-Perot cavity resonatorsare provided with a waveguide disposed between and terminated byreflectors. Branches (i.e., waveguides) extending from the Y-branchsplitter are adjacent the cavity resonators and directional couplers areutilized to couple light from the branches to the Fabry-Perot cavities.These waveguides, which are evanescently side-coupled to the resonatorsby the directional couplers, respectively, will, at certain wavelengths,show resonant reflection from the cavity. These waveguides may beterminated such that light moving forward (away from the Y-junction) isscattered out. Such termination could be for example a taper or aninverse-taper of the waveguide.

In order to analyze device performance and functionality, a3-dimensional model was used. In this model, the waveguides are 400 nmwide by 210 nm tall and are characterized by a refractive index of 3.48.These waveguides are placed in an ambient medium with a refractive indexof 1.45. TE wave propagation is assumed. The waveguides support a singleTE mode.

The reflectors are implemented using waveguides with an array ofrectangular bumps with a period of 323 nm placed symmetrically on eachside. The bumps are 283 nm wide by 210 nm tall by 142 nm deep. Eachreflector is 27 μm long. The stop band of these reflectors extends from˜1525 nm to ˜1575 nm. The cavity waveguides and bumps both have arefractive index of 3.48. For each cavity coupling region, the inputwaveguide and cavity waveguide are parallel for a length of 5 μm andspaced 320 nm edge-to-edge. The bends in the input waveguides have aradius of 7 μm. As discussed above, in a particular implementation, thewaveguide sections for the two cavities have lengths of 149.3 μm and138.1 μm, respectively. Utilizing phase control sections, the effectivelength of the cavities can be tuned as described more fully herein. Thedifferent sizes of the cavities provide different free spectral rangesas well as shifting of the resonance peaks, which enable reflection fora single peak when the cavities are combined using a Y-junction.

FIG. 8A is a cross-sectional diagram of the waveguide structure at input110. As illustrated in FIG. 8A, the device structure can be fabricatedon a silicon-on-insulator (SOI) platform including a silicon substrate810, an insulator layer 812, and silicon layer (e.g., a single crystalsilicon layer 814). In this embodiment, the buffered oxide layer 812 is2 μm thick, but can vary in thickness from about 0.1 μm to about 50 μm.The silicon waveguide 820 along with other devices is etched fromsilicon layer 814. SiO₂ is then deposited via one or more methods suchas PECVD, CVD, sputtering, SACVD, or the like. In the illustratedembodiment, the silicon layer has a thickness of 210 nm, but this valuecan range from about 50 nm to about 5,000 nm. The width of the waveguideis 400 nm in order to support single mode operation, but could benarrower or wider depending on the particular application.

FIG. 8B is a cross-section diagram of a waveguide structure at couplingregion 122. As illustrated in FIG. 8B, the fabrication on asilicon-on-insulator (SOI) platform is shown. The buffered oxide layer812 is 2 μm thick as illustrated in FIG. 8A, but may vary as appropriateto the particular application. The waveguides are etched from siliconlayer 814 and then SiO₂ is deposited via method such as PECVD, CVD,sputtering, SACVD, or the like. In the illustrated embodiment, thewaveguide 820 is the extension of waveguide 820 illustrated in FIG. 8A.Waveguide 822, separated from waveguide 820 by 320 nm, is the waveguidedisposed in the resonant cavity 120/130. The spacing between waveguidesenables evanescent coupling and can vary from about 10 nm to about 5,000nm depending on the coupling coefficient desired. Although thethicknesses of the waveguides in this region (i.e., 400 nm) areillustrated, this is not required and other thicknesses can be utilized.As an example, the waveguide thickness can vary as a function of lengthin the case of tapered waveguides.

FIG. 1B is a plot showing reflectance spectra for cavities of thetunable reflective structure illustrated in FIG. 1A and FIG. 1C is aplot showing reflectance spectra for the entire tunable reflectivestructure illustrated in FIG. 1A. The operation of the wavelengthtunable reflectors based on the coupled cavity configuration illustratedin FIG. 1A is illustrated by FIGS. 1B and 1C. As shown in the figures,using the Vernier effect, the combined cavity structure exhibits asingle, tunable reflectance peak at the input to the Y-junction. Thereflectance spectra for the double side-coupled cavity reflectorstructures is illustrated, with the reflectance associated with thefirst side coupled cavity 120 being represented by a solid line and thereflectance associated with the second side coupled cavity 130 beingrepresented by the dashed line. As will be evident to one of skill inthe art, the differing cavity lengths will result in differingreflectance spectra. The reflectance spectrum for the combined cavitiesis illustrated in FIG. 1C, with a single spectral mode having highreflectance (e.g., approaching unity) and all remaining modes having lowreflectance (e.g., less than 0.4). In some embodiments, multiplespectral modes are present in the spectra for the combined cavity, withthe single mode shown in FIG. 1C providing particular benefits in someapplications.

In the configuration illustrated in FIG. 1A, the coupling region of theside-coupled cavities is designed so that 13% of the power in the inputwaveguide couples to the cavity waveguide in a single pass. Weakcoupling leads to a higher cavity quality factor and narrower reflectionpeaks. Of course, other coupling coefficients can be used. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

It should be noted that the evanescent side coupling illustrated in theside-coupled cavities 120 and 130 in FIG. 1 are not required by thepresent invention. As an alternative, a Y-junction splitter can be usedto replace the illustrated evanescent side-coupling (illustrated in FIG.4A). In this case, 50% of the power in the input waveguide couples tothe cavity in a single pass in an embodiment. In other embodiments, thecoupling coefficient can vary.

In order to tune the reflectance spectra of the individual cavities, andthereby, the combined cavity, a phase adjustment section 122 is providedfor the first side-coupled cavity and a phase adjustment section 132 isprovided for the second side-coupled cavity. In some embodiments, aseparation is provided between the phase adjustment (i.e., phasecontrol) sections and the reflectors to provide for temperaturestabilization in the reflectors. Thus, in addition to variation inreflectance spectra as a result of the differing lengths, control can beexerted over the index of refraction in the waveguides, resulting intuning of the reflectance spectra for the combined cavity. In someimplementations, the index control is accomplished using thermalcontrol, for example, by heating or cooling some or all of the centralportion of the cavity structures, or alternatively heating up the wholecavity including the mirrors. In other implementations, carrier-basedcontrol is provided, for example, by integrating diode structures inconjunction with the waveguides. Additional description related tocarrier-based control of index is provided in U.S. patent applicationSer. No. 13/605,633, filed on Sep. 6, 2012, entitled “Tunable HybridLaser with Carrier-Induced Phase Control,” the disclosure of which ishereby incorporated by reference in its entirety for all purposes.

FIG. 1D is a plot showing spectral tuning of the reflectance spectra forcavities of the tunable reflective structure illustrated in FIG. 1A, andFIG. 1E is a plot showing tuning of the reflectance spectra for theentire tunable reflective structure illustrated in FIG. 1A. Referring toFIG. 1D, the response of the reflection spectrum for one of theside-coupled cavities is illustrated for a first phase condition (i.e.,index of refraction of the waveguide region is a first value) and for asecond phase condition (i.e., index of refraction of the waveguideregion is a second, different value). The phase control can beimplemented using either or both phase controller 122 or 132 asdiscussed above.

As the index of refraction associated with the cavity varies as afunction of the phase control, the reflection spectra for the combinedcavity also changes, with the single mode peak shifting to a different(higher or lower) wavelength in comparison with the wavelength of theresponse of the (a) the individual side-coupled cavities and (b) theentire structure shown in FIG. 1A. This shifts the spectrum so that thereflection at the input peaks at a new wavelength that is blue-shiftedor red-shifted with respect to the original position of the peak,depending on the particular control algorithm. Thus, the peak can bescanned over a variety of wavelengths using embodiments of the presentinvention. Tuning is achieved by shifting the phase of the two cavitiessuch that two peaks associated with the individual cavities overlap at apredetermined wavelength. Then fine tuning of the single peak can beachieved by changing the index of the two cavities together, thus movingthe wavelength at which the peak occurs.

As shown in FIG. 1E, the wavelengths at which the peak reflectanceoccurs may be tuned by changing the refractive index of the cavitywaveguides. This phase adjustment may be achieved by heating thewaveguides, by using electro-optic devices, or by other suitabletechniques. The phase of the reflection peaks from each side cavitydepends on the cavity orientation and the order of the cavity resonance.As an example, if a cavity waveguide center is aligned with the centerof the coupling region of the input waveguide, as shown in FIG. 1A, andthe reflection is observed within the input waveguide at the couplingregion center, then the reflection peaks will have phase Mπ, where m isan integer denoting the order of the resonance.

In using the Vernier effect with the configuration illustrated in FIG.1A, it may occur that both side cavities reflect strongly at a givenwavelength, but with a phase difference of π. Therefore, phase shifter140 is provided in one arm of the Y-branch. When phase shifter 140 turnson, a round-trip phase π is added to ensure proper matching at theY-junction. In the case where the cavities reflect with the same phase,the phase shifter will be turned off.

FIG. 2A is a simplified plan view of a tunable reflector 200 accordingto an embodiment of the present invention. As illustrated in FIG. 2A, adouble Fabry-Perot loop is provided that includes two Fabry-Perotcavities, FP cavity 210 and FP cavity 220. The cavities are formed usingsidewall modulated mirrors in this embodiment as illustrated in theinset of FIG. 2A, but other reflective structures, including DFB gratingstructures, could be utilized to form the cavities. For the tunablereflector illustrated in FIG. 2A, the loop includes the two coupledFabry-Perot cavities with partially transmitting mirrors. The loop ischaracterized by a high reflectance value at wavelengths for which bothcavities are resonant.

An input is provided to the tunable reflector at input waveguide 205,which is coupled to Y-splitter 207. The branches splitting off from theY-junction connect to form a loop. In some embodiments, the Y-junctioncoupler can be a multimode interference coupler, directional coupler, orother suitable splitter. Within the loop, three partially transmittingmirrors 212, 215, and 222 are placed, forming two coupled Fabry-Perotcavities with different lengths. In an embodiment, the lengths are 230microns for FP cavity 210 and 190 μm for FP cavity 220, but these arejust exemplary values and other lengths can be utilized, for example,lengths ranging from about 5 μm to about 1000 μm. In some embodiments,the center mirror 215 can be replaced by two mirrors, each associatedwith one of the FP cavities.

In an embodiment, the distances between the two outer mirrors and theY-junction differ by a half effective wavelength with respect to thecenter wavelength of the operation band, or integer multiples of thatdifference. As a result, light reflected by the outer mirrorsdestructively interferes at the Y-junction and does not reflect back tothe input. Using the Vernier effect, the combined cavity structure canexhibit a single reflectance peak when both cavities are resonant (i.e.,fully transmitting).

In order to tune the reflectance peak, phase control sections 218 and228 are provided in the waveguides of both FP cavities. It should benoted that in some embodiments, a single phase control section isutilized. The wavelengths at which peak reflection occurs may be tunedby changing the refractive index of the waveguides disposed between themirrors, using the phase control sections 218 and 228. Phase control canbe implemented by heating the waveguides, by using electro-opticdevices, by carrier-induced index changes, or the like.

In summary, each Fabry-Perot resonator in the coupled system has afrequency comb associated with it and the waveguide sections control thefree spectral range (FSR). When both elements are transmitting (i.e., atresonance in case of a Fabry-Perot cavity) the entire device isreflecting. The reflectance peaks are controlled by tuning the effectivelength of the FP cavities (e.g., by heating, current injection, or othermechanism to modify the optical path length), which changes the point ofoverlap of the 2 combs (i.e., the Vernier effect). Thus, embodiments ofthe present invention provide a tunable reflector that may be used for acompact feedback device for a laser (replacing, for example, gratingreflectors). More than two cavities and combs can be used in otherimplementations.

As an alternative implementation, the FP cavities, each with tworeflectors, can be replaced by Mach-Zehnder interferometers (MZI). Inthis implementation, similar concepts are applicable, but theinterference element is different (i.e., MZI instead of Fabry-Perot).Each MZI creates a wavelength comb and both MZIs can be tuned forwavelength selection using the

Vernier effect. Creating the combs with MZIs could be accomplished bymaking the arms with different lengths. The interfering elements can bedifferent in other implementations, the lengths of the arms can bedifferent, the splitting can be done in several ways, and there can be atotal of one or more interfering elements.

FIG. 2B is a plot showing reflectance spectra as a function of indexshift for the tunable reflector illustrated in FIG. 2A, demonstratingthe device functionality. The plot illustrated in FIG. 2B was generatedfor 300 nm wide waveguides with a refractive index of 2.62 disposed inan ambient medium with a refractive index of 1.45. TM wave propagationis assumed. The waveguides support a single TM mode. This corresponds toeffective index method representation of a three dimensional system witha TE mode.

The reflectors 212, 215, and 222 are implemented using waveguides with asinusoidal modulation of the waveguide width (i.e., sidewall modulatedmirrors or modulated waveguide structures). They have a refractive indexof 2.62. The modulation amplitude is 30 nm on each side, and themodulation period is 416 nm. The outer reflectors 212 and 222 are each50 μm long and the center reflector 215 is 100 μm long. The stopband ofthese reflectors extends from ˜1544 nm to ˜1551.5 nm. The distancebetween reflectors for the first cavity is 190 μm and is 230 μm for thesecond cavity. The Y-junction splitters are made of circular waveguidesections of inner and outer radii of 14.7 μm and 15 μm, respectively.These values are merely exemplary and the present invention is notlimited to these particular values. As examples, the modulationamplitude can vary from about 5 nm to about 150 nm. The length of theouter reflectors can vary from about 5 μm to about 1,000 μm. The lengthof the center reflector can vary from about 5 μm to about 1,000 μm. Oneof ordinary skill in the art would recognize many variations,modifications, and alternatives.

Referring to FIG. 2B, reflectance values for three different phasecontrol values for phase control section 218 are illustrated. For thefirst difference in index change in the 230 μm long cavity, the singlereflectance peak is positioned at ˜1546 nm, for the second change inindex, the single reflectance peak is positioned at ˜1548 nm, and forthe third change in index, the single reflectance peak is positioned at˜1550 nm. Of course, depending on the particular index differenceproduced by the phase control sections, the reflectance peak will shiftin wavelength. In other embodiments, the phase control section 222, orboth phase control sections can be utilized to produce the wavelengthtuning

FIG. 2C is a plot showing reflectance spectra over a second range forthe tunable reflective structure illustrated in FIG. 2A. As illustratedin FIG. 2C, the Fabry-Perot loop exhibits a strong reflectance valueoutside the stopband of the reflectors. As discussed more fully inrelation to FIG. 3A, the addition of a Y-branch terminated with areflector can reduce these reflections outside the stopband of thereflectors.

FIG. 3A is a simplified plan view of a tunable reflector 300 accordingto an alternative embodiment of the present invention. As illustrated inFIG. 3A, a double Fabry-Perot loop is formed within a Y-junction cavity,providing another configuration for a tunable reflector. Strongreflection occurs for wavelengths within the stopband of the gratingreflector 350 on the bottom left portion of the device. In thisembodiment, the Fabry-Perot loop forms one end of a cavity, while areflector 350 is placed at the other end of the cavity. The inputwaveguide 305 is coupled to this cavity with a Y-junction 310, althoughthe coupling may be realized using a directional coupler, MMI, or othersuitable coupler. Strong reflection at the input waveguide occurs whenthe double FP loop 320 is resonant within the stopband of reflector 350.In order to provide tunability, a phase shift element, such as a heateror electro-optic device, is placed in the Y-junction cavity to ensurethe resonant condition is met when strong reflectance is desired. Thereflector 350 terminating the reflecting branch of the Y-junction has afinite stopband, so the device can exhibit strong reflectance forwavelengths within this stopband. The reflector 350 can be fabricatedusing a periodic grating or other suitable reflective structure.

In comparison with the FP loop illustrated in FIG. 2A, the embodimentillustrated in FIG. 3A provides strong reflection into the input at atunable single peak, with a reduced reflection outside the stopband.This is useful, for example, in laser feedback applications and insystems in which lasing is desired only inside a narrow wavelengthrange.

FIG. 3B is a plot showing reflectance spectra for the tunable reflectorillustrated in FIG. 3A. As shown in FIG. 3B, reflections outside thereflector stopband are reduced in comparison with the reflectancespectra shown in FIG. 2C.

In computing the reflectance spectra shown in FIG. 3B, the modelutilized for the structure shown in FIG. 2A was modified so the outputof the Fabry-Perot Loop 320 connects to a Y-junction 310. One branch ofthe Y-junction is a 52 μm long waveguide terminated by a 200 μm longgrating reflector 350. The reflector can be a sidewall modulatedwaveguide similar to those used in the Fabry-Perot Loop. Otherreflective structures can also be utilized. The second branch of theY-junction is the input waveguide 305. As discussed above, because theresponse outside the mirror stopband is suppressed, this device issuitable for use in a variety of applications including tunable filtersfor communication and for laser feedback.

FIG. 5 is a simplified flowchart illustrating a method of operating atunable reflector according to an embodiment of the present invention.The method includes directing light into an input/output port to provideinput light (510) and splitting the input light into a first portion anda second portion (512). The method also includes waveguiding the firstportion of the input light through a first resonant cavity (514) andwaveguiding the second portion of the input light through a secondresonant cavity (516). As an example, the resonant cavities can beFabry-Perot cavities with sidewall modulated mirrors or gratingreflectors. The method further includes coupling light transmittedthrough the first resonant cavity into the second resonant cavity (518)and coupling light transmitted through the second resonant cavity intothe first resonant cavity (520). The loop structure provided byembodiments of the present invention enables the production of highreflectance at wavelengths for which both cavities are resonant. Itshould be understood that in embodiments in which a single common mirroris used, for example, mirror 215 in FIG. 2A, waveguiding a portion ofthe light through the first cavity and then coupling light transmittedthrough the first cavity into the second cavity is a process that occursconcurrently as light exiting mirror 215 from one of the cavities is, asa result of the geometry of the structure, coupled into the othercavity.

In an embodiment, at least one of the first resonant cavity or thesecond resonant cavity includes a phase control element such as a heateror a carrier-based (e.g., current injection) device that can vary theindex of refraction of the waveguide, enabling the ability to controlthe reflectance spectrum of the device. Splitting the input light caninclude the use of an optical splitter coupled to the input/output port,for example, a directional coupler, a Y-branch splitter, a multimodeinterference (MMI) coupler, or the like. In some embodiments, the firstresonant cavity and the second resonant cavity share a common reflector,whereas in other embodiments, the resonant cavities utilize independentreflectors, such as grating reflectors. As an example, the firstresonant cavity and the second resonant cavity can utilize sidewallmodulated waveguide reflectors. The cavity length of the first resonantcavity can differ from a cavity length of the second resonant cavity.Coupling into the cavities can also be accomplished with eitherY-junctions, directional couplers, MMIs, or the like.

It should be appreciated that the specific steps illustrated in FIG. 5provide a particular method of operating a tunable reflector accordingto an embodiment of the present invention. Other sequences of steps mayalso be performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 5 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

In an alternative embodiment to that illustrated in FIG. 1A, theY-junction splitters may be replaced by waveguide directional couplerswith a predetermined splitting ratio to achieve a similar or identicalfunctionality. The coupling into the branches of the splitter may beequal or may not be equal and, accordingly, unequal power may betransmitted into the branches. As described herein, reflectors mayinclude grating structures, such as sidewall modulated waveguides, orother suitable reflective structures. If periodic grating reflectors areused, significant reflection occurs only at wavelengths within thegrating stopband. Wavelengths outside this stopband will be transmittedthrough the gratings, where light may be scattered out using a suitablewaveguide termination so as to avoid reflection.

FIG. 4A is a simplified plan view of a multi-cavity reflector 400according to an embodiment of the present invention. As illustrated inFIG. 4A, two Y-junction cavities are utilized, each having a distinctfree spectral range. The combination of the two Y-junction cavitiesproduces a single reflectance peak. As discussed with the otherimplementations, tuning of the cavities can be performed to tune thewavelength of the reflectance peak.

In the illustrated embodiment, the input is provided at waveguide 410.The top cavity 420 includes two reflectors 422 and 424, separated by adistance of 93 μm. Of course, this distance is merely exemplary and thedistance between the reflectors can range from about 10 μm to about1,000 μm, for example, 153 μm. The reflectors 422 and 424 can be gratingstructures, sidewall modulated waveguides, or the like. In theillustrated embodiment, the lateral spacing between reflector 424 andthe input waveguide is 30 μm although this can vary as well. The topcavity 420 is coupled to the input by a primary arm 414 that is coupledto a primary propagation section 421 disposed between reflector 422 (afirst primary reflector coupled to the primary propagation section) andreflector 424 (a second primary reflector). The primary propagationsection 421 is coupled to reflector 424 using a primary Y-junctionwaveguide 426 coupled to the primary propagation section 421. Theprimary Y-junction 426 is disposed at the half-way point between thereflectors 422 and 424. This position is exemplary and the Y-junction426 may meet the primary propagation section 421 at any suitable point.Reflector 422 is also coupled to the primary Y-junction waveguide 426.

The bottom cavity 430 includes two reflectors 432 and 434, separated bya distance of 59 μm. Of course, this distance is merely exemplary andthe distance between the reflectors can range from about 10 μm to about1,000 μm, for example, 93 μm. The reflectors 432 and 434 can be gratingstructures, sidewall modulated waveguides, or the like. In theillustrated embodiment, the lateral spacing between reflector 434 andthe input waveguide is 30 μm although this can vary as well. Asdiscussed in relation to the top cavity, the bottom cavity 430 iscoupled to the input/output by a secondary arm 416 and includes asecondary propagation section 431, a first secondary reflector 432coupled to the secondary propagation section 431, a secondary Y-junctionwaveguide 436 coupled to the secondary propagation section 431, and asecond secondary reflector 434 coupled to the secondary Y-junctionwaveguide 436. The secondary Y-junction 436 is disposed at the half-waypoint between the reflectors 432 and 434. This position is exemplary andthe Y-junction 436 may meet the secondary propagation section 431 at anysuitable point.

Referring to FIG. 4A, light enters the reflective structure at the input410 and is split by the coupler 412 (e.g., a Y-junction, a 3 dB coupler,an MMI, or the like) and passes to reflectors 422 and 432. Lightreflected by reflector 422 propagates to reflector 424 and is reflectedto reflector 422. Light reflected by reflector 432 propagates toreflector 434 and is reflected to reflector 432. In some embodiments,suitable phase adjustments are provided using phase control section 450to introduce a variable index of refraction in a predetermined region asappropriate in conjunction with the coupler.

After the second reflection from reflectors 422 and 432, the reflectedlight propagates through the Y-junction 412 as illustrated along thereflection direction in the figure. As discussed below, changing therefractive index in one or more of the legs of the device results in ashift in the combined reflectance from ˜1545.5 nm to ˜1548 nm. As willbe evident to one of ordinary skill in the art, multiple reflections canoccur between the illustrated reflectors and the discussion providedabove, is merely provided to illustrate the optical propagation path.

FIG. 4B is a plot showing reflectance spectra for individual andcombined cavities as illustrated in FIG. 4A. As illustrated, each of theindividual cavities is characterized by a multi-peak reflectancespectra. For example, the top cavity 420 includes peaks at ˜1544 nm,˜1545.5 nm, and ˜1547 nm. The bottom cavity 430 includes peaks at˜1545.5 nm and ˜1548 nm. The combined cavity includes a singlereflectance peak at ˜1545.5 nm.

Referring once again to FIG. 4A, phase control section 450 is providedin one of the cavities, in this case, in the waveguide of the top cavity420. It should be noted that the phase control section could be disposedin the bottom cavity 430 or both cavities can implement phase control.FIG. 4C is a plot showing reflectance spectra for individual andcombined cavities with phase control according to an embodiment of thepresent invention. By introducing a change in the refractive index ofthe top cavity equal to 0.1%, the peak of the combined cavity can bered-shifted by 2.5 nm, from ˜1545.5 to ˜1548 nm. Thus, tuning of thesingle reflectance peak is provided by the embodiment illustrated inFIG. 4A. This index change is exemplary and other suitable values can beutilized to obtain a desired wavelength shift. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

Although various embodiments of the present invention have beendescribed in relation to FIG. 1A, FIG. 2A, FIG. 3A, and FIG. 4A, it willbe appreciated that the elements utilized in the devices, includingY-junctions or directional couplers, can be interchanged, with opticalelements from one embodiment utilized in place of optical elementsdescribed in relation to other embodiments. Additionally, as an example,Y-junctions or directional couplers could be alternatively replaced byMMIs. Thus, the optical elements, including Y-junction splitters,directional couplers, and MMIs, can appear in any combination in theconfigurations illustrated herein and used for both the first splitterand the coupling mechanisms to the cavities.

FIG. 6 is a simplified flowchart illustrating a method of providingvariable spectral reflectance according to an embodiment of the presentinvention. The method includes inputting input light at an input/outputport (610) and splitting the input light into a first portion and asecond portion using a Y-junction waveguide (612). In an embodiment, thefirst portion passes through a region of variable index of refraction.As an example, a portion of the waveguide can include a phase controlelement such as a heater or a carrier-based (e.g., current injection)device that can vary the index of refraction of the waveguide, enablingthe ability to control the reflectance spectrum of the device.

The method also includes reflecting the first portion from a firstreflector (614), waveguiding the reflected first portion through aY-junction waveguide to reflect from a second reflector (616) andreflecting light reflected from the second reflector from the firstreflector to provide a first spectral reflectance signal (618). Themethod further includes coupling the first spectral reflectance signalto the input/output port (620), reflecting the second portion from athird reflector (622), and waveguiding the reflected second portionthrough a Y-junction waveguide to reflect from a fourth reflector (624).Additionally, the method includes reflecting light reflected from thefourth reflector from the third reflector to provide a second spectralreflectance signal (626) and coupling the second spectral reflectancesignal to the input/output port (628). In an embodiment, the firstspectral reflectance signal and the second spectral reflectance signalcomprise a combined signal.

It will be appreciated that embodiments of the present invention provideresonant structures in which there are multiple cavities and, therefore,the nature of the resonant cavities involves a large number ofreflections back and forth between the cavity mirrors depending on thecavity quality factors. Thus, the description provided herein inrelation to light traveling along a given path is provided merely toillustrate device operation and should not limit the number or extent ofpropagation throughout the structures. Accordingly, the discussion ofpropagation of rays should be understood in the context of the cavitiesin which the rays are propagating.

It should be appreciated that the specific steps illustrated in FIG. 6provide a particular method of operating a combined cavity according toan embodiment of the present invention. Other sequences of steps mayalso be performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 6 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 7 is a simplified schematic diagram illustrating a laser 700incorporating reflective structures provided by embodiments of thepresent invention. As illustrated in FIG. 7, a gain medium 710, pumpedby a pump source 712, is disposed in a resonant cavity formed byreflectors 720 and 730. The pump source can be integrated with the gainmedium in some embodiments, for example, as found in a semiconductorlaser gain medium pumped by electrical current. The gain medium canprovide a pulsed gain profile or a CW gain profile as appropriate to theparticular application. Optical elements 714 and 716 are provided in thecavity to perform mode control and the like. In some embodiments, theoptical elements can be discrete elements such as lenses or integratedelements such as waveguides. Thus, an integrated package for the laser700 is included within the scope of the present invention.

Reflector 720 and/or reflector 730 can be implemented using any of thereflective structures discussed herein, including tunable reflectivestructure 100, tunable reflector 200, tunable reflector 300, ormulti-cavity reflector 400. In a particular embodiment, the reflectorsare implemented in an SOI wafer, with the gain medium integrated using ahybrid bonding process, thereby providing an integrated package for thelaser 700. An alternative embodiment provides an amplifier (e.g., adouble pass amplifier) in which one of the reflectors is removed toprovide for a double pass through the gain medium. Additionalapplications include the use of the reflectors (which can provide anarrow band reflectance) in other optical devices including, withoutlimitation, interferometers, spectrometers, or the like. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A reflective structure comprising: aninput/output port; an optical splitter coupled to the input/output port,the optical splitter having a first branch and a second branch; a phaseshifter in the first branch of the optical splitter; a first resonantcavity optically coupled to the first branch of the optical splitter,wherein the first resonant cavity comprises a first set of reflectorsand a first waveguide region disposed between the first set ofreflectors; a second resonant cavity optically coupled to the secondbranch of the optical splitter, wherein the second resonant cavitycomprises a second set of reflectors and a second waveguide regiondisposed between the second set of reflectors; and a phase controlsection in the first waveguide region or in the second waveguide region.2. The reflective structure of claim 1 wherein the phase control elementcomprises at least one of a heater or a carrier-based element.
 3. Thereflective structure of claim 1 wherein the optical splitter comprises adirectional coupler.
 4. The reflective structure of claim 1 wherein thefirst set of reflectors and the second set of reflectors comprisemodulated waveguide structures.
 5. The reflective structure of claim 1wherein a length of the first waveguide region and a length of thesecond waveguide region are different.
 6. The reflective structure ofclaim 1 wherein the first resonant cavity and the second resonant cavitycomprise Fabry-Perot cavities.
 7. The reflective structure of claim 1wherein a reflector, of the first set of reflectors or of the second setof reflectors, is 27 microns long.
 8. The reflective structure of claim1 wherein a reflector, of the first set of reflectors or of the secondset of reflectors, has a stop band that extends from ˜1525 nanometers to˜1575 nanometers.
 9. The reflective structure of claim 1 wherein thefirst branch or the second branch has a bend with a radius of curvatureof 7 microns.
 10. The reflective structure of claim 1 wherein the firstbranch or the second branch has a refractive index of 3.48.
 11. Thereflective structure of claim 1 wherein the first resonant cavity has awaveguide section having a length of 149.3 microns and the secondresonant cavity has a waveguide section having a length of 138.1microns.